doc/man7/EVP_KDF-KRB5KDF.pod

   1 =pod
   2
   3 =head1 NAME
   4
   5 EVP_KDF-KRB5KDF - The RFC3961 Krb5 KDF EVP_KDF implementation
   6
   7 =head1 DESCRIPTION
   8
   9 Support for computing the B<KRB5KDF> KDF through the B<EVP_KDF> API.
  10
  11 The EVP_KDF-KRB5KDF algorithm implements the key derivation function defined
  12 in RFC 3961, section 5.1 and is used by Krb5 to derive session keys.
  13 Three inputs are required to perform key derivation: a cipher, (for example
  14 AES-128-CBC), the initial key, and a constant.
  15
  16 =head2 Identity
  17
  18 "KRB5KDF" is the name for this implementation;
  19 it can be used with the EVP_KDF_fetch() function.
  20
  21 =head2 Supported parameters
  22
  23 The supported parameters are:
  24
  25 =over 4
  26
  27 =item "properties" (B<OSSL_KDF_PARAM_PROPERTIES>) <UTF8 string>
  28
  29 =item "cipher" (B<OSSL_KDF_PARAM_CIPHER>) <UTF8 string>
  30
  31 =item "key" (B<OSSL_KDF_PARAM_KEY>) <octet string>
  32
  33 These parameters work as described in L<EVP_KDF(3)/PARAMETERS>.
  34
  35 =item "constant" (B<OSSL_KDF_PARAM_CONSTANT>) <octet string>
  36
  37 This parameter sets the constant value for the KDF.
  38 If a value is already set, the contents are replaced.
  39
  40 =back
  41
  42 =head1 NOTES
  43
  44 A context for KRB5KDF can be obtained by calling:
  45
  46  EVP_KDF *kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
  47  EVP_KDF_CTX *kctx = EVP_KDF_new_ctx(kdf);
  48
  49 The output length of the KRB5KDF derivation is specified via the I<keylen>
  50 parameter to the L<EVP_KDF_derive(3)> function, and MUST match the key
  51 length for the chosen cipher or an error is returned. Moreover the
  52 constant's length must not exceed the block size of the cipher.
  53 Since the KRB5KDF output length depends on the chosen cipher, calling
  54 L<EVP_KDF_size(3)> to obtain the requisite length returns the correct length
  55 only after the cipher is set. Prior to that B<EVP_MAX_KEY_LENGTH> is returned.
  56 The caller must allocate a buffer of the correct length for the chosen
  57 cipher, and pass that buffer to the L<EVP_KDF_derive(3)> function along
  58 with that length.
  59
  60 =head1 EXAMPLES
  61
  62 This example derives a key using the AES-128-CBC cipher:
  63
  64  EVP_KDF *kdf;
  65  EVP_KDF_CTX *kctx;
  66  unsigned char key[16] = "01234...";
  67  unsigned char constant[] = "I'm a constant";
  68  unsigned char out[16];
  69  size_t outlen = sizeof(out);
  70  OSSL_PARAM params[4], *p = params;
  71
  72  kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
  73  kctx = EVP_KDF_new_ctx(kdf);
  74  EVP_KDF_free(kdf);
  75
  76  *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_CIPHER,
  77                                          SN_aes_128_cbc,
  78                                          strlen(SN_aes_128_cbc));
  79  *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
  80                                           key, (size_t)16);
  81  *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_CONSTANT,
  82                                           constant, strlen(constant));
  83  *p = OSSL_PARAM_construct_end();
  84  if (EVP_KDF_set_params(kctx, params) <= 0)
  85      /* Error */
  86
  87  if (EVP_KDF_derive(kctx, out, outlen) <= 0)
  88      /* Error */
  89
  90  EVP_KDF_free_ctx(kctx);
  91
  92 =head1 CONFORMING TO
  93
  94 RFC 3961
  95
  96 =head1 SEE ALSO
  97
  98 L<EVP_KDF(3)>,
  99 L<EVP_KDF_free_ctx(3)>,
 100 L<EVP_KDF_ctrl(3)>,
 101 L<EVP_KDF_size(3)>,
 102 L<EVP_KDF_derive(3)>,
 103 L<EVP_KDF(3)/PARAMETERS>
 104
 105 =head1 HISTORY
 106
 107 This functionality was added to OpenSSL 3.0.
 108
 109 =head1 COPYRIGHT
 110
 111 Copyright 2016-2020 The OpenSSL Project Authors. All Rights Reserved.
 112
 113 Licensed under the Apache License 2.0 (the "License").  You may not use
 114 this file except in compliance with the License.  You can obtain a copy
 115 in the file LICENSE in the source distribution or at
 116 L<https://www.openssl.org/source/license.html>.
 117
 118 =cut
 119